Simplest is a part like UNL2003 or ULN2803.Connect the + of your strips to a +12V supply, connect the - of hte strip to the output of the ULN2003.ULN2003 Gnd connects to same Gnd that 12V supply and Arduino connect to. COM is not connected.Arduino connects to the ULN2003 input - when input is high, output goes low, strip turns on.